One way to produce an alkenyl bridged ring compound is to use a Diels-Alder reaction, which is the additive reaction of an olefin with a cyclic diolefin. For example, 5-vinyl-2-norbornene (an alkenyl bridged ring compound) may be produced by the Diels-Alder reaction of 1,3 butadiene (an olefin) with 1,3-cyclopentadiene (a cyclic diolefin). 5-vinyl-2-norbornene (VNB) is very desirable as it is an intermediate in the production of 5-ethylidene-2-norbornene (ENB), a termonomer used in the production of ethylene-propylene-diene monomer (EPDM) rubbers.
The conditions necessary to bring about the Diels-Alder reaction of 1,3-butadiene (BD) with 1,3-cyclopentadiene (CPD) are well known in the art. In particular, BD may be contacted with CPD in the liquid phase at temperatures of from 100.degree. to 200.degree. C. and a pressure of from 150 to 300 psi (1,034-2,068 kPa). It is not necessary to use a catalyst to advance the Diels-Alder reaction. The reaction is generally completed in 0.1 to 100 hours and is usually conducted under an inert atmosphere. Preferably, the reaction may be conducted in a liquid state, most preferably in a liquid-full reaction vessel.
When making VNB the preferred reaction occurs between BD and CPD, however, undesirable polymerization reactions may also occur. For example, BD monomers may react with each other to form polymers. Similar polymerization reactions may occur between CPD monomers. Undesirable by-products include dimerization of BD to form 4:vinyl-1-cyclohexene (VCH), rearrangement of VNB to form 4, 7, 8, 9-tetrahydroindene (THI) and Diels-Alder adducts of BD or CPD with VNB, VCH, or THI (hereinafter referred to as "trimers of BD/CPD").
CPD will readily react with itself to form dicyclopentadiene (DCPD). In turn, DCPD will crack upon heating, back to the CPD monomer. Therefore, both CPD and/or DCPD can be used as the cyclic diolefin feedstock in the production of alkenyl bridged ring compounds. As used herein, the term "(di)cyclopentadiene" refers to cyclopentadiene, dicyclopentadiene or mixtures thereof in the reaction mixture. In the present application, cyclopentadiene and dicyclopentadiene are regarded as the same substance in the calculation of the conversion of raw materials and, hence, the transformation of cyclopentadiene into dicyclopentadiene, and vice versa is not considered a conversion.
Certain compounds are known which suppress or inhibit the undesired polymerization reactions. Any one or combination of these inhibitor compounds may be added to the reactants in order to produce more VNB from the same amount of starting material and to avoid plugging certain parts of the reaction apparatus with the high molecular weight polymers which might otherwise be formed. Many inhibitor compounds are known in the art including 2,6-di-t-butyl-p-cresol, diphenyinitrosamine, and N-substituted p-phenylenediamines.
In addition to the problems caused by fouling, the undesired by-products are difficult to separate from VNB which create additional problems in the production process of making VNB.
As taught by U.S. Pat. No. 3,728,406, improved selectivity during the production of VNB, or any alkenyl bridged ring compounds, can be achieved by two methods. First, the starting material may be limited to the CPD monomer, rather than the DCPD dimer. This results in higher conversion of the (di)cyclopentadiene to VNB. Second, the reaction can be discontinued early after only a relatively small portion of the total CPD and/or DCPD has been consumed. One of the disadvantages of this process is that a substantial amount of residual DCPD is left in the reactor effluent. However, this residual DCPD can be recovered to produce additional alkenyl bridged ring compound. Steps to recover the DCPD include: (1) vacuum distillation to separate the DCPD from the THI and trimers of BD/CPD, or (2) selectively cracking the DCPD to CPD, which is then easily separated from the THI and trimers of BD/CPD because it is much more volatile. Unfortunately, either method requires additional processing equipment and the conditions used to crack DCPD typically result in fouling of the equipment.
It would be advantageous if a simple and economical method could be devised to recover the residual dimer of the cyclic diolefin from the production of an alkenyl bridged ring compound, by cracking the dimer to cyclic diolefin monomer, and recycle the monomer to produce more of the desired alkenyl bridged ring compound without fouling of the equipment.